Thin film light guides or wave guides are planar elements intended for utilization in optical circuitry or other optoelectronic devices which require light transmission as part of their operation. The developments in the field of such thin film devices have led to the belief that thin film technology will eventually lead to integrated optical circuits and devices comparable to those currently available based on microelectronic integrated electrical circuits.
In order to achieve a thin film wave guide or light guide, a layer of transparent material having a higher index of refraction must be flanked by materials having a lower index. This can be done by making sandwiched layers of low-high-low index, or by placing a layer or film of material on a substrate with a lower index (i.e. glass) and having the upper surface exposed to the surrounding media (i.e. air) having a lower index. The materials typically used for thin film devices have been metal or semiconductive oxides and nitrides such as silicon dioxide, silicon nitride, aluminium oxide, titanium dioxide and zinc oxide to name a few. Some older research focussed on organic polymers polymerized from monomers, but this technique has not been widely developed by the art. Semiconductor or inorganic films have been deposited by various means, a common one of which is PECVD or plasma enhanced chemical vapor deposition. This process is typically used with inorganic materials, which are deposited onto substrates at relatively high temperatures (above 150.degree. C.). Polymer film layers have been studied as potential optical waveguides.
One area of early research was the use of organic materials to make optical films. Such organic films may be deposited by glow-discharge polymerization, or "plasma polymerization," but the process is significantly different from PECVD, particularly in its temperature requirement, since this process may be performed at room temperature (25.degree. C.) without heating of the substrate. The plasma polymerization method can yield films free of holes or discontinuities and which are mechanically and chemically tougher than typical conventional polymers.
As described above, if a film of material of homogeneous index of refraction is to be used as a wave guide, it must have a higher index than the surrounding materials. This is necessary to that light approaching the boundaries of index decrease is reflected back into the film of the wave guide, so that the light propagates down the film. If the material is not homogenous as to wider of refraction, i.e. has multiple layers or varying material, then it is desirable to have a region of higher index somewhere in the interior of the film which confines the light. A smoothly varying index of refraction is the most desirable since reflections from sharp discontinuities in index lose more energy than occurs in the path bending of light which occurs with a smoothly graded index charge. Of the methods previously known for producing optical thin films, few can produce materials with smoothly varying index of refraction.
Therefore, the prior art is generally cognizant of the concept of making polymer thin films by deposition of monomers from a plasma onto a substrate. For example, a system of making such a thin film light guide circuit is described in U.S. Pat. No. 3,822,928. In the method for fabricating such circuitry disclosed in that patent, layers of different refractive indices are created by forming the thin polymer films from vapors of two separate monomers which are plasma polymerized in a common plasma and deposited jointly on the substrate. By varying the relative proportions between the two monomers, the relative proportions of the monomers' contribution to the overall polymeric film can be varied. The variation in the constituent monomers in the film creates a difference in index of refraction. Thus, the refractive index desired in any particular layer is achieved by making variations of the proportions between the two monomers used to create the film as a part of the fabrication process.